White LED solid lighting that rose in the early 21st century is commonly known as semiconductor lighting and has been developed rapidly due to such advantages as high light efficiency, good energy-saving property, no pollution, environmental protection, long service life, etc.
After years of efforts, white LED will be pushed into the area of general (indoor) lighting, general white LED bulb were commercially available from 2009 to 2012, and even straight-tube LED fluorescent lamps were also available on the market. They all need to meet the strict parameters in China Standard and U.S. Energy Star standard (which came into force on Aug. 31, 2010). In addition to light efficiency, LED's colorimetric parameters (color temperature, chromaticity coordinate, color rendering property) are particularly important. It is quite urgent to develop a white LED with different color temperature, high color rendering property and high light efficiency. At present, the demand on high-color-rendering lighting of LED cannot be met and achieved by traditional manufacturing of a white LED only through a YAG:Ce yellow fluorescent material, this is because this light source is under a shortage of red component and has low color rendering index. A top-quality red fluorescent material is needed for implementing the white LED regardless of use of a semiconductor blue light LED chip or use of combination of a near ultraviolet light LED chip and a photoconversion fluorescent material.
In summary of various fluorescent materials that can be used for white LED (p 321-329 in Luminescence and Luminescent Materials edited by X U, Xurong, S U, Mianzeng, published on October 2004, Chemical Industry Press), LIU, Xingren indicates that, there is particularly a shortage of red fluorescent material with excellent performances at present.
To improve the color rendering index (Ra) of white LED, Ce3+ and Pr3+ co-excited YAG (See P326 in Luminescence and Luminescent Materials; H. S. Jang, et al, J. Lumin. 2007, 126: 126) and TAG:Ce(Tb3Al5O12:Ce) fluorescent material (See A. A. Setlur et al, Proc. of SPIE, 2004, Vol. 5187: 142; M. Nazarov et al, J. Solid State Chem, 2007, 180: 2493) have been acquired by research on the basis of YAG:Ce yellow powder. The former is characterized by superposition of a weaker 611 nm (Pr3+) red emission line on the emission spectrum of Ce3+, and the latter is characterized by little movement of the emission spectrum of Ce3+ towards long wave. These measures aim at increasing the red component a little in the emission spectrum of Ce3+, which contributes to improving the color rendering index Ra of LED. However, this scheme is at the cost of brightness reduction, furthermore, improvement of the color rendering index Ra is quite limited and cannot meet the demands of a white LED with high color rendering property and medium/low color temperature. The (Ca, Sr)S:Eu2+ red fluorescent material can be effectively excited by a blue light LED to emit fresh red light and can be used in a white LED with high color rendering property, but such an alkaline earth sulfide that is very poor in chemical stability absorbs moisture and is decomposed to generate toxic hydrogen sulfide gas, thereby blackening and damaging the fluorescent material. In addition, this fluorescent material cannot be used in a near ultraviolet light (NUV) white LED due to its low NUV excitation efficiency.
It has also been proposed that the well-known Y2O3:Bi, Eu fluorescent material is used as a red material for NUV white LED (U.S. Pat. No. 6,255,670B1), so does the Y2O2S:Eu red fluorescent material for television (Taguchi Tsunemasa et al, J. Lingt, Vis. Env. 2003, 27(3):131), however, they have a common problem of low luminous efficiency under the excitation of NUV (390-410 nm) and blue light.
Recently, Eu2+-excited nitride and nitrogen oxide red fluorescent materials have been successfully developed to bring excellent effect, such as MSi5N8:Eu(M:Ca, Sr, Ba), CaAlSiN3:Eu2+ and the like. But these red fluorescent materials are unstable in the air above 200° C., and special, inaccessible and dangerous raw materials as well as a high-temperature (1600-2000° C.) high-pressure special synthesis method are needed in preparation of such red fluorescent materials, which imposes a limitation to their development. Moreover, such nitrogen oxide fluorescent materials are extremely expensive, which restricts their application in white LED.